The present invention relates to flexible film for packaging articles therein having a dienophile additive blended into a polyvinylidene chloride barrier layer. More specifically, the present invention relates to a flexible film having a plurality of layers including a barrier layer having properties that prevent molecular diffusion of gases and/or vapors. The barrier layer may be made from a copolymer of polyvinylidene and an alkyl acrylate, such as, for example, methyl acrylate. The dienophile additive may protect the barrier film from degradation caused by heat, light, such as UV radiation, and/or electron beam irradiation of the film structure.
It is, of course, generally known to utilize a multilayer film structure having a plurality of desirable properties. For example, one of the layers in the multilayer film structure may be a barrier layer that prevents or otherwise restricts molecules of a gas, a vapor or a flavor of a food article from diffusing therethrough. The barrier layer may be a xe2x80x9ccorexe2x80x9d of the multilayer film structure and may be surrounded by a number of other film layers, each of the layers having a plurality of characteristics. For example, an abuse layer may be provided on an outside of the film structure for adding a property that causes the film to resist tearing, scratching and/or cracking. Additionally, a sealant layer may be provided on an alternate surface of the film structure for providing a layer that may seal to itself or to other layers or articles upon heating. Further, multiple layers may be contained within the film structure having a plurality of xe2x80x9ctiexe2x80x9d or adhesive layers for bonding the internal layers, such as the barrier layer, to the abuse layer, the sealant layer or any other layer within the multilayer film structure.
A typical barrier layer may include a copolymer of polyvinylidene chloride (xe2x80x9cPVdCxe2x80x9d) and an alkyl acrylate, such as, for example, methyl acrylate (xe2x80x9cMAxe2x80x9d). This flexible film formulation is typically referred to as MA-Saran and is produced by the Dow Chemical Company (xe2x80x9cDowxe2x80x9d). The addition of methyl acrylate to PVdC provides improved thermal stability to the PVdC, as PVdC itself is relatively unstable at temperatures required for producing a film via, for example, blown-film or extrusion lamination. General information on PVdC, MA-Saran and other related films including properties thereof can be found in xe2x80x9cVinylidene Chloride and Poly(vinylidene Chloride)xe2x80x9d Kirk-Othmer: Encyclopedia of Chemical Technology, 3rd Ed., Vol. 23 (New York: John Wiley and Sons, Inc. 1983), pp.764-798.
Although PVdC is relatively useful due to its low permeability to gases and vapors such as oxygen and water vapor, its thermal instability results in degradation at temperatures at or around the melt temperatures of the PVdC-MA. Further, irradiation of the film layers for cross-linking of polymer chains within adjacent layers of the multilayer films may cause PVdC to degrade as well. Light, such as UV radiation, may also cause the PVdC to degrade. The degradation reaction may produce HCl as a by-product along with the formation of a conjugated polyene. While the addition of the alkyl acrylate reduces the degradation process, heat and/or radiation still may cause significant degradation.
The degradation reaction generally proceeds as follows:
(CH2CCl2)nxe2x86x92(CHxe2x95x90CCl)n+nHCl 
In addition to the production of hazardous by-products such as HCl, the degradation may also cause a decrease in the crystallinity of PVdC polymer or copolymer structure, thereby increasing the potential for gas or vapor transmission therethrough. Therefore, the heat necessary to extrude PVdC or bond other layers to the barrier layer, as well as radiation utilized to cause cross-linking, may lower the quality of PVdC as a barrier material.
In addition, the formation of conjugated polyenes causes a film produced by PVdC to discolor from clear to yellow. If enough degradation occurs, PVdC film may turn brown or black. Specifically, optical properties of the film are greatly reduced due to the degradation of PVdC by heat, light or electron beam irradiation.
As noted in the Kirk-Othmer article, stabilizing PVdC is fairly developed. Specifically, dienophiles such as, for example, maleic anhydride and dibasic lead maleate, have been found to prevent discoloration of PVdC films by reacting with and stabilizing the color-producing conjugated dienes within the polymer. Dienophiles generally stabilize these conjugated polyenes by reacting with the double bonds in multiple Diels-Alder reactions. The reactions remove the conjugated double bonds, thereby improving the properties of the film, especially optical clarity. A further advantage of using a dienophile is that HCl remains within the film thereby slowing the progress of the reaction.
However, formulations of PVdC with dienophiles besides the ones mentioned above have rarely been utilized up to this point. U.S. Pat. No. 5,679,465 to Bekele teaches using a dienophile that is a copolymer having an anhydride moiety. Specifically, Bekele discloses a dienophilic terpolymer having olefinic, acrylic, and anhydride comonomers, or a grafted copolymer of maleic anhydride. Further, a preferred embodiment of Bekele includes an ethylene/alkyl acrylate/maleic anhydride terpolymer, such as ethylene/butyl acrylate/maleic anhydride terpolymer. However, dienophilic copolymers are difficult to use and may interfere with the crystallinity of the PVdC film matrix, thereby decreasing the barrier properties of the film. Further, cheaper and more effective dienophiles, along with processes of combining the dienophiles with the PVdC formulation are necessary.
Therefore, a need exists for an improved PVdC formulation having dienophiles incorporated therein for flexible film packaging that will react with conjugated polyenes formed by the degradation of PVdC by heat, light and electron beam irradiation. Further an improved flexible film package and method of manufacturing the same is needed.
The present invention relates to a flexible film comprising a layer of polyvinylidene chloride and a dienophile additive for reacting with the conjugated polyene degradation product to improve the barrier properties and optical properties of the film. Further, a package for an article and a method of manufacturing the same are also provided.
To this end, in an embodiment of the present invention, a flexible film is provided comprising a plurality of layers wherein a first layer comprises polyvinylidene chloride and a dienophile additive in the first layer of polyvinylidene chloride.
In another embodiment of the present invention, the dienophile additive is selected from the group consisting of a maleate, a cinnamate, and maleic anhydride.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% by weight ethyl trans-cinnamate.
In another embodiment of the present invention, the dienophile additive is between about 0.05 % and about 5 % by weight dibutyl maleate.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% by weight methyl trans-cinnamate.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% by weight dimethyl maleate.
In another embodiment of the present invention, the dienophile additive is between about 0.05 % and about 5% by weight maleic anhydride.
In another embodiment of the present invention, the polyvinylidene chloride is a copolymer with methyl acrylate.
In another embodiment of the present invention, the flexible film includes an outer layer disposed on a first surface of the film structure and a sealant layer disposed on a second surface of the film structure.
In another embodiment of the present invention, the flexible film includes an outer adhesive layer for bonding the first layer of polyvinylidene chloride and the dienophile to the outer layer and an inner adhesive layer for bonding the first layer of polyvinylidene chloride and the dienophile to the sealant layer.
In another embodiment of the present invention, the outer layer and the outer adhesive layer are polymer blends of linear low density polyethylene and ethylene-vinyl acetate copolymer
In another embodiment of the present invention, the inner adhesive layer is linear low density polyethylene.
In another embodiment of the present invention, the sealant layer is a copolymer of ethylene-vinyl acetate.
In an alternate embodiment of the present invention, a flexible film package for a food article is provided. The flexible film package includes a flexible film having a plurality of layers contained therein wherein a first layer comprises polyvinylidene chloride and a dienophile additive wherein the dienophile is selected from the group consisting of a maleate, a cinnamate and maleic anhydride.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% ethyl trans-cinnamate.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% dibutyl maleate.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% methyl trans-cinnamate.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% dimethyl maleate.
In another embodiment of the present invention, the dienophile additive is between about 0.05% and about 5% by weight maleic anhydride.
In another embodiment of the present invention, the first layer is made from a copolymer of polyvinylidene chloride and methyl acrylate.
In another embodiment of the present invention, the flexible film package includes outer layers of the flexible film wherein the outer layers include an abuse layer and a sealant layer and adhesive layers between the outer layers and the first layer.
In another embodiment of the present invention, the abuse layer is a polymer blend of linear low density polyethylene and ethylene-vinyl acetate copolymer.
In an alternate embodiment of the present invention, a method of making a flexible film includes providing polyvinylidene chloride, blending a dienophile with the polyvinylidene chloride wherein the dienophile is selected from the group consisting of a cinnamate, a maleate, and maleic anhydride, and extruding the polyvinylidene chloride/dienophile blend through a die to make a film.
In another embodiment of the present invention, the polyvinylidene chloride/dienophile blend is coextruded with a plurality of melt streams to produce a plurality of layers.
In another embodiment of the present invention, between about 0.05% and about 5% by weight of the dienophile is blended with the polyvinylidene chloride.
It is, therefore, an advantage of the present invention to provide a flexible film, a package and a method of manufacturing the same that includes a dienophile additive for reacting with conjugated double bonds within the polymer structure.
Further, an advantage of the present invention is to provide a flexible film, a package and a method of manufacturing the same that improves the barrier properties of the film. Still further, an advantage of the present invention is to provide a flexible film, a package and a method of manufacturing the same that improves the optical properties of the film.
And, an advantage of the present invention is to provide a flexible film, a package and a method of manufacturing the same that blends the dienophile additive with PVdC prior to extrusion of the film. In addition, an advantage of the present invention is to provide a flexible film, a package and a method of manufacturing the same that provides for continuous improvements to the barrier film for a time after the film has been extruded and irradiated.
An additional advantage of the present invention is to provide a flexible film, a package and a method of manufacturing the same that reacts with the conjugated polyenes of the degradation product but leaves HCl within the system to slow the progression of the degradation reaction.
Additional features and advantages of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments and from the drawings.